Every few years, when the nation goes through one of its paroxysms over immigration, there are promises to tighten borders, deal with illegal immigrants already here and crack down on employers.

Such crackdowns, however, are mostly myth. The law is enforced only in fits and starts.

Enforcement plummeted beginning in the late 1990s. Authorities arrested about 14,000 illegal immigrants at work sites in 1998. Arrests declined by 80 percent the next year — and kept on dropping, to a total of 445 in 2003, according to Congress’ nonpartisan Government Accountability Office.

Even if the government wanted to emphasize work-site arrests, it doesn’t have the manpower. The Department of Homeland Security has about 325 agents to contend with 7.2 million illegal workers.

The long-range plan was to repeat the experiment industry by industry and state by state. This, authorities concluded, would persuade employers that it’s more economical to hire legal workers, slowing illegal immigration. This would be done without disruptive raids or wholesale deportations.